Bubbler-type liquid sources are often used in chemical reactors, such as semiconductor processing tools. Examples include chemical vapor deposition (CVD) and atomic layer deposition (ALD) reactors. Bubblers tend to exhibit better efficiency than so-called “overflow” sources, in which vaporization depends on heating and the carrier gas simply flows over the surface of liquid, rather than being bubbled through the liquid. The efficiency of the overflow sources depends greatly on flow conditions. On the other hand, bubbler type sources are not always safe. Under certain conditions there is risk that the pressure inside the source bottle is higher than in the inlet feeding tube (which is submerged in the liquid source). The higher pressure pushes the liquid precursor material into the carrier gas inlet of the feeding tube. This uncontrolled situation is a safety and contamination risk. This type of risk is higher with ALD reactors due to the fact that the operation is based on pulsing and there is typically more than one route for the gases into the reactor. These conditions increase the risk of accidentally pumping the gas inlet tubing to a lower pressure than the liquid container.
A source container 10 shown in FIG. 1 is positioned in a temperature-controlled environment 12. The container 10 has a bottle inlet 20 for the inactive carrier gas and a bottle outlet 22 for the gas mixture generated in the source container 10. A manual inlet isolation valve 14 and an outlet isolation valve 16 are used for isolating the container 10 from the surrounding conduits and room atmosphere when a depleted container is replaced with a refilled one. The source container has a refill port 18 that is used for adding liquid source chemical to the container, and in some arrangements the port is provided in fluid communication with an auto-refiller to maintain a minimum level of liquid source. A computer-controlled three-way valve 24 directs inactive gas either to the container 10 or to a by-pass line 90.
A computer-controlled source valve 26 is used for regulating the flow of gas mixture from the container 10 to the reaction space 52. A computer-controlled by-pass valve 28 is used for purging the inlet 20 and outlet 22 of the residual source chemical vapor before replacing a depleted container 10 with a fresh one. The by-pass valve 28 is kept closed during deposition processes. The computer controlled valves 24, 26, 28 are placed in a temperature-controlled environment 30 to prevent the condensation of the source chemical vapor at these valves.
A reaction chamber 50 defines a reaction space 52 in which a substrate 54 is housed. The reaction space 52 is in fluid communication with chemical sources, including the liquid source container 10, through a source conduit 92, and in fluid communication with a vacuum pump 70 through an exhaust conduit 72. The vacuum pump has an outlet 74 for gases. A back-suction conduit 96 is in fluid communication with the exhaust conduit 72 and in fluid communication with the source conduit 92 at a junction 98 that is placed between the reactor 56 and the computer-controlled source valve 26. A back-suction restrictor 104, e.g. a capillary, restricts the flow rate of gases through the back-suction conduit 96 to the exhaust conduit 72.
A by-pass restrictor 100, e.g. a capillary, restricts the flow rate of inactive gas from the three-way valve 24 through the by-pass conduit 90 that is connected to the source conduit 92 at a connection point 94. The connection point 94 is placed in the source conduit 92 between the reaction chamber 50 and the junction 98 of the back-suction conduit 96. Flow rate restrictors 106, 108, e.g. capillaries, are placed in the source conduit 92 on both sides of the by-pass conduit connection point 94 to form a gas diffusion barrier volume between the said flow rate restrictors 106, 108. The flow rate restrictors 100, 104, 106, 108 are shown inside temperature-controlled environments 30, 56 to enable fast purging of the conduits 90, 92.
At least two source chemicals (one shown in FIG. 1) are connected to the reaction chamber 50 and are alternately pulsed into the reaction space 52 during an ALD process. The source chemical pulses are preferably separated with inactive gas flow that purges the reaction space, although other means are known for keeping the reactant gases separate spatially and temporally. A typical ALD pulsing sequence consists of four basic process steps: reactant A pulse, purge A, reactant B pulse, purge B. The pulsing sequence is repeated as many times as is needed for obtaining a thin film of desired thickness. In other arrangements, the pulsing sequence can be more complicated.
Referring to FIG. 1, during a reactant A pulse, an inactive gas (e.g., nitrogen or argon) flows from an inactive gas source 40 through a mass flow controller 42. The three-way valve 24 guides the inactive gas to the source container 10. Evaporated source chemical vapor diffuses with the inactive gas inside the source container 10 and flows as a gas mixture to the outlet 22. The source valve 26 allows the gas mixture to flow to the reaction space 52 where the source chemical molecules chemisorb on the substrate 54 surface until available reactive surface sites have been filled with the molecules and the chemisorption process saturates, leaving no more than one molecular layer of the source chemical molecules or their chemisorbed fractions on the surface.
During a purge A step, the source valve 26 is kept closed and the three-way valve 24 guides the flow of the inactive gas through the by-pass conduit 90 to the gas diffusion barrier volume that is located in the vicinity of the connection point 94. The inactive gas flow divides into two parts with the help of the flow rate restrictors 106, 108. About 90% of the inactive gas flows to the reaction space 52 and purges the residual reactant A away from the reaction space 52 to the exhaust conduit 72. About 10% of the inactive gas flows backwardly through the source conduit 92 to the junction point 98 of the back-suction conduit 96 and then the gas flows through the back-suction conduit 96 to the exhaust conduit 72 and finally to the vacuum pump 70. The backward flow makes sure that source chemical molecules do not diffuse along the source conduit 92 to the reaction space 52 during the purge period.
It will be understood that certain problems related to the liquid source are created with the source pulsing method. If the source container 10 is a bubbler and the three-way valve malfunctions, it is possible that the gas pressure at the inlet 20 to the container 10 becomes smaller than the gas pressure inside the container 10. In that case the bubbler pressure tends to push some liquid toward the gas inlet 20 and even further upstream of the container 10. The by-pass conduit 90 can become thereby contaminated with the source chemical and the reactor does not operate in ALD mode any longer.
As shown in FIG. 2a, a bubbler tube 200 extends into the source container 10 so that the tip 202 of the bubbler tube is near the bottom of the source container 10. The tip 202 is thus below the surface 220 of the liquid source chemical. During a source chemical pulse, inactive gas is fed to the inlet 20, flows through the manual inlet isolation valve 14, through the bubbler tube 200 and forms bubbles 206 that rise to the surface of the liquid 220. Source chemical molecules diffuse into the inactive gas, forming a gas mixture, and the gas becomes more or less saturated with the source chemical vapor. The gas mixture leaves the gas space 208 of the container 10 through the manual outlet isolation valve 16 and continues through the outlet 22 and further to the reaction space (not shown).
Referring to FIG. 2b, in case of a malfunction, the pressure of the inactive gas at the inlet 20 may become smaller than the pressure of the gas mixture in the gas space 208 of the container 10. The pressure pushes the liquid surface 220 and forces liquid back through the bubbler tube 200 so that the liquid surface 222 in the bubbler tube 200 creeps towards the inlet 20 and upstream areas of the source conduits. This is problematic, especially in liquid sources that undergo rapid pressure variations during ALD processing. A backlash valve (not shown) placed at the inlet of the container is not entirely fail-safe because of corrosive source chemicals.
This problem has partially been alleviated with the use of an “overflow” source, shown in FIGS. 3a and 3b, rather than a bubbler, but it creates certain new problems. Inactive gas flows through the inlet 20 into the container 10. An inlet conduit is arranged to have its tip 300 always above the liquid surface 220 so that back-flow of the liquid upstream of the source is not possible. The inactive gas mixes with the source chemical vapor and the mixture enters a flow space 302 (arranged coaxially with the inlet in the illustrated example) and then the mixture flows through the manual outlet isolation valve 16 through the outlet 22 and further to the reaction space (not shown).
When the liquid is gradually consumed, the surface 220 of the liquid lowers and the distance between the tip 300 of the inlet conduit and the liquid surface 220 increases. When the distance becomes large, as shown in FIG. 3b, it takes a longer time for the source chemical molecules to mix with the inactive gas and the gas mixture becomes more dilute as deposition proceeds. It can be understood that the decreasing source chemical concentration creates problems with the dosage of the source chemical into the reaction space. Since the dosage per timed pulse decreases over time, either an overlong pulsing period is employed, which would waste time and reactant during initial phases, or the pulsing period must be increased over time, which is impractical for manufacturers to implement in process recipes that should be consistent from run to run. Rather than risking underdosage, the tendency is to overdose. ALD processes are not particularly sensitive to the dosage, since the surface reactions are self-saturating such that deposited layers remain uniform despite overdosage. Nevertheless it may be difficult to control the amount of overdosage of the source chemical and rather a lot of chemical is wasted.
Thus, there is a need for an improved liquid source that addresses at least some of the problems described above.